With the widespread of smaller and lighter weight electronic equipment such as cell phones and notebook computers in recent years, demand is increasing for higher capacity secondary batteries as the power sources therefor. Hitherto, non-aqueous electrolyte secondary batteries having: a positive electrode containing, as a positive electrode active material, a lithium cobalt oxide such as LiCoO2; and a negative electrode containing a carbon material have been developed and are now widely used.
LiCoO2, however, is very costly because it contains Co. For this reason, various metal oxides are proposed as an alternative to LiCoO2 and are being vigorously studied. Examples of such metal oxides include: lithium composite metal oxides containing nickel as an essential element (hereinafter sometimes referred to as nickel-containing composite oxides) such as LiNiO2 and LiNi1-xCoxO2 obtained by partially replacing Ni in LiNiO2 with Co; and LiMn2O4.
Positive electrodes containing a nickel-containing composite oxide as the active material, in particular, can offer a higher energy density than those containing a lithium cobalt oxide as the active material. The use of such positive electrodes, therefore, reduces the cost for producing non-aqueous electrolyte secondary batteries and enhances the capacity of the batteries.
In the production of a positive electrode for secondary batteries, usually, a material mixture slurry is applied on a current collector, followed by drying and rolling to form a positive electrode material mixture layer. The material mixture slurry is prepared by adding a positive electrode active material to a liquid obtained by dissolving or dispersing a binder in an organic solvent, which is then stirred and mixed. As the binder, polyvinylidene fluoride (hereinafter referred to as PVDF), polytetrafluoroethylene and an ethylene-propylene-diene copolymer are usually used. Among them, PVDF is the most suitable material for the binder because it is not easily dissolved in electrolytes and has superior electrolyte retention capability.
Fluorocarbon resin having a vinylidene fluoride unit has low resistance to alkali. It is thus accompanied by a problem that when the material mixture slurry contains an alkali salt, the slurry is gelated to cure the material mixture layer.
Lithium composite metal oxides as typified by LiCoO2 and LiNiO2 usually contain an alkali salt which is introduced therein during the production process. It is difficult to remove such an alkali salt completely. The synthesis of LiNiO2, in particular, requires an excessive amount of alkali salt, the alkali salt is thus likely to remain.
The remaining alkali salt facilitates the gelation of the material mixture slurry as well as helps cure the coating film (i.e., positive electrode material mixture layer). The mechanism of this phenomenon is considered as follows. PVDF first contacts a lithium salt contained in the active material, causing dehydrofluorination (HF) reaction of the PVDF to form a carbon-carbon double bond. Because this double bond is extremely unstable, a crosslinking reaction occurs between or within the molecules of the PVDF. As this reaction proceeds with time, the slurry is gelated or the material mixture layer is cured. If a spirally wound electrode group is produced using a positive electrode having the cured material mixture layer, a crack occurs in the material mixture layer during the spiral winding process, which might break the positive electrode.
In order to prevent the positive electrode material mixture layer from curing, partial replacement of PVDF with a rubber-like acrylic copolymer (preferably in an amount of 45 wt % or more) is proposed (see e.g., Japanese Laid-Open Patent Publication No. 2000-21407). The acrylic copolymer for use is, for example, a copolymer of styrene-butadiene rubber (SBR) added with methyl methacrylate, itaconic acid or acrylonitrile unit. In order to improve the flexibility of a positive electrode plate, Japanese Laid-Open Patent Publication No. 2003-331825 proposes to use a PVDF derivative and rubber particles at a weight ratio of 1:3 to 3:1 as the binder.